Method of producing clad porous metal articles



June 30, 1964 H. A. KUCHEK 3,138,856

METHOD OF PRODUCING cum POROUS METAL ARTICLES Filed Oct. 9, 1961 Henryfl. Kuche/r HTTORNEY United States Patent 3,138,856 METHOD OF PRODUCING CLAD POROUS METAL ARTICLES Henry A. Kuchek, Auburn, Mich., assignor to The Dow Chemical Company, Midland, Mich., a corporation of Delaware Filed Oct. 9, 1961, Ser. No. 143,621 2 Claims. (Cl. 29-423) This invention relates to a method of producing a clad, porous article of manufacture, useful as a filter.

In the accompanying drawings, forming a part of this specification, and in which like numerals are employed to designate like parts throughout the same,

FIGURE 1 is a section view of a portion of the final product of this invention,

FIGURE 2 is a section view of an extrusion compact, from which the final product of FIGURE 1 is made, and

FIGURE 3 is a View of a small section of the salt filled metal casting which forms the core of the extrusion compact shown in FIGURE 2.

The end product of the present invention, illustrated in FIGURE 1, has a thin dense metal exterior surface 4 and a porous metal core 5. Core metal 5 is of the nature of a metal sponge and has pores which are interconnected, longitudinally elongated, and of a generally uniform, relatively small diameter in the plane normal to the longitudinal axis of the article.

The article produced by the method of this invention is particularly useful as a rigid filter for separating solids from liquids, but the invention is not thus limited. In use as a filter, the fluid material to be filtered enters either end of the article. The solids are entrapped in the porous core and the filtrate is recovered from the other end of the article.

Preparation of the article of this invention is begun by casting a suitable metal or metal alloy, preferably aluminum or aluminum alloy containing at least 75 percent aluminum into a mold filled with a water soluble salt in granular form. Suitable sizes of granular salt are those as small as will pass a 100 mesh screen and those as large as will allow suflicient metal to remain to result in a self-supporting article, sometimes as much as an inch in diameter or greater, depending on the size of the mold. While the shape of the salt will preferably be equiaxed, oblong or irregularly shaped pieces may be employed. The salt must have a melting temperature higher than the temperature at which the metal or metal alloy is to be cast, must be insoluble in the liquid metal or metal alloy, and must not be so strongly or rapidly corrosive in aqueous media toward the metal or metal alloy as to destroy the metal or metal alloy in the time required to leach the salt from the casting, and must not be reducible in aqueous media by the cast metal or metal alloy so as to leave an undesired metal residue in the porous core. Salts usable in the method of this invention are, for example, sodium chloride, potassium chloride, barium chloride, and the like which meet the requirements described above.

By proper selection of the salt to be employed in the casting step, keeping in mind the requisite properties described above, the practice of this invention may be extended to any extrudable metal or metal alloy, as for example lead, magnesium, aluminum, tin, copper, and their alloys.

Prior to casting, the salt-filled mold is preheated to a temperature near the melting point of the metal or metal alloy, but without melting the salt. The molten metal is then poured into the hot salt-filled mold. Preheating the mold ensures a good penetration by the molten metal 3,138,856 Patented June 30, 1954 Ice of the spaces between the salt grains and is particularly desirable if the salt grains are of a very small size.

The salt-filled metal casting is allowed to cool and solidify and is then removed from the mold. The salt granules 6, as shown in FIGURE 3, are in contact with each other so that when they are removed, the voids left are interconnecting. Water is then employed to leach the salt 6 from the casting, leaving a metal sponge. The voids or pores left in the metal 7 by the leaching process are then filled with a low melting salt. This may be accomplished by immersing the leached casting in a bath of molten salt or mixture of salts until the pores of the metal sponge are filled with said salt mixture. After the pores are filled, excess salt mixture should be removed from the surface of the casting.

The low melting salt must have a melting point above the temperature at which the metal or metal alloy which forms the sponge is extruded, and below the melting temperature of said metal sponge. The low melting salt mixture should be as inert to the metal as the salt used in the casting step. Salts usable in the method of this invention are often mixtures having lower melting points than their constituents, for example, potassium chloridelithium chloride, and numerous others known to physical chemists, which meet the requirements described above.

Referring to FIGURE 2, the salt mixture filled metal sponge 7 is placed in a jacket or sleeve 8 of a metal or metal alloy similar to that forming the porous metal, but in the normal dense and non-porous condition. The inner dimension of the jacket should be such as to provide a relatively close fit around the salt-filled metal sponge and make good metal to metal contact. with the exterior surface of the sponge. The jacket should have a wall thickness appropriate for the desired final thickness after extrusion. The salt filled metal sponge 7 forms the core of the article, and the jacket or sleeve 8 forms the solid outer surface of the article.

If desired, the leached casting may be placed in the cladding jacket prior to immersion in the low-melting salt. If this sequence is preferred, any residual salt may be removed from the jacket surface prior to the extrusion step of the process.

Salt-containing metal 7, encased by the jacket 8 is hereinafter called an extrusion compact.

In order to compress and tighten the jacket 8 around the porous metal, increase the overall length, decrease the overall diameter, and elongate the pores, the extrusion compact is extruded. Generally an extrusion reduction ratio of at least 2 to 1 is accomplished, with ratios of 36 to 1 being preferred, and ratios as high as to 1 may be employed in some instances.

After extrusion, the extruded article is heated to a temperature above the melting point of the low melting salt mixture within the core of the article. The molten salt mixture is then removed from the core of the particle by any suitable means, such as air pressure applied to one end of the article, centrifugal force, draining, or the like. Residual salt mixture may be removed by leaching with Water. Water leaching is accomplished by forcing water through the article. This is continued until substantially all the salt has been leached from the porous metal core. At this point the operation is essentially complete.

During the extrusion step, the salt-containing pores are elongated in the direction of extrusion, which direction is parallel to the longitudinal axis of the article. Said saltcontaining pores are also reduced in diameter in the plane normal to the longtiudinal axis of the article. The salt is retained within the pores during extrusion to prevent collapse of the pore walls within the core.

Good uniformity of pore diameter is obtainable with this method. Said method has an advantage in that fairly coarse salt grains may be used in casting even through a fine pore diameter may be desired in the final product. If it were necessary to use salt grains of fine diameter, problems in casting might arise. The core metal would have difiiculty in filling the. voids uniformly, so that after leaching large gaps might appear in the porous structure. By proper selection of the size and shape of the water soluble salt grains used in casting, the extrusion reduction ratio, and the thickness of the cladding jacket, a large number of pore sizes and shapes may be obtained with a substantial degree of reproducibility.

The jacket is tightly bound to the core by the pressures of the extrusion process so that an article of essentially unitary construction is finally obtained.

The following example is set forth to illustrate, but not to limit, this invention.

Example A cylindrical mold having a diameter of 3 inches and a depth of 8 inches was prepared by filling it with sodium chloride granules of an 8l2 mesh size, U.S. sieve series. The sodium chloride containing mold was preheated to about 700 C. An aluminum alloy having a nominal composition of percent silicon, balance aluminum was cast at a temperature of about 760 C. into the mold.

After solidification, the metal-salt cylinder was removed from the mold and the salt was leached from the casting with water. After leaching was complete, the cylinder was fitted into a metal sleeve having a inch Wall thickness and having a nominal composition of 0.4 percent silicon, 0.7 percent magnesium, and the balance aluminum to form an extrusion compact.

At a temperature of about 450 C., the extrusion compact was immersed in a molten mixture of about equal parts of lithium chloride and potassium chloride to fill the pores within the core of the extrusion compact. After cooling, the exterior surface of the metal jacket was cleaned of residual salt mixture by means of a water rinse.

After preheating the salt-filled extrusion compact to about 250 C., the unit was subjected to an extrusion process. A 500 ton extrusion press with a conical die having a face set 45 from the line of extrusion and a /2 inch opening was used at an extrusion reduction ratio of 36 to 1 and a speed of about 3 feet per minute. After extrusion, the /2 inch diameter extrude was cut into 3 foot lengths and the 3 foot lengths were heated to about 400 C. in order to melt the salt mixture in the pores. Air pressure of about pounds per square inch was applied to one end of each length of extrude to force the molten salt from the pores within the core of the extrude. Residual salt was leached from the pores with water forced through the core of the extrude.

An examination of the thus-prepared article showed that the cladding jacket had a thickness of about inch, there was a good bond between the cladding jacket and the porous metal, and the pores were elongated, interconnecting, and of uniform small diameter.

When the article produced by the method of this invention is used as a fluid filter, suspended solids are removed from the fluid to a high degree of efficiency. Cleaning of the filter may be accomplished by simply flushing, suitably in the reverse direction of flow, with a clear, inert liquid.

The article produced by the method of this invention has good structural characteristics. It is self-supporting without the basic weakness common to unclad metal sponge of like dimensions. Further, the porous core is protected from damage which may occur in handling, shipping, etc.

Various modifications may be made in the present invention without departing from the spirit or scope thereof, and it is to be understood that I limit myself only as defined in the appended claims.

I claim:

1. The method of producing a clad, porous, metal article which comprises casting a metal into a mold filled wtih water soluble salt granules which have a melting point above the temperature at which the metal is cast, removing the salt-metal casting from the mold after solidification, leaching the salt from the casting, filling the pores thus-formed with a water soluble salt which melts at a temperature below the melting point of the metal, inserting the salt filled metal casting into a dense metal jacket, subjecting the assembly thus-formed to an extrusion process and removing the salt.

2. The method of producing a clad, porous, metal article which comprises casting a metal of the class of aluminum and aluminum alloys into an elongated mold of regular cross section filled with water soluble salt granules which have a melting point above the temperature at which the metal is cast, removing the salt-metal casting from the mold after cooling, leaching the salt from the casting, filling the pores thus-formed with a water soluble salt which melts at a temperature below the melting point of the metal, inserting the salt-filled metal casting into a dense metal jacket of a metal of the class of aluminum and aluminum alloys, subjecting the assembly thus-formed to an extrusion process, heating the extrusion product to a temperature above the melting point of the salt in the porous core and below the melting temperature of the metal, forcing air through the article under pressure to remove the molten salt, and finally, forcing water through the article to remove any residual salt from the porous core by leaching.

References Cited in the file of this patent UNITED STATES PATENTS 872,621 Gray Dec. 3, 1907 2,628,417 Peyches Feb. 17, 1953 2,747,742 Royer et a1 May 26, 1956 2,754,972 McCann July 17, 1956 2,983,597 Elliott May 9, 1961 2,986,810 Brick June 6, 1961 3,006,044 Mayer Oct. 31, 1961 FOREIGN PATENTS 468,790 Italy Dec. 31, 1962 

1. THE METHOD OF PRODUCING A CLAD, POROUS, METAL ARTICLE WHICH COMPRISES CASTING A METAL INTO A MOLD FILLED WITH WATER SOLUBLE SALT GRANULES WHICH HAVE A MELTING POINT ABOVE THE TEMPERATURE AT WHICH THE METAL IS CAST, REMOVING THE SALT-METAL CASTING FROM THE MOLD AFTER SOLIDIFICATION, LEACHING THE SALT FROM THE CASTING, FILLING THE PORES THUS-FORMED WITH A WATER SOLUBLE SALT WHICH MELTS AT A TEMPERATURE BELOW THE MELTING POINT OF THE METAL, 